1. Field of the Invention
The present invention relates to a screen printing apparatus and a screen printing method which print a pattern with paste on the surface of a substrate which is aligned with a mask in which predetermined pattern openings are formed.
2. Related Art
In a mounting process of electronic components, screen printing of supplying a pasted conductive material (conductive paste, hereinafter simply referred to as ‘paste’) onto the surface of a substrate is used frequently in order to mechanically and electrically join electronic components to the substrate. The screen printing includes supplying paste onto the surface of a mask provided with pattern openings formed in a predetermined pattern in a state where the mask is brought into surface contact with the surface of a substrate, and applying and spreading the paste on the surface of the mask by means of a squeegee which slides on the surface of the mask, thereby printing a pattern on the surface of the substrate with the paste stagnated in the pattern openings. Electronic components are placed on the part where a pattern is printed with the paste, and the electronic components are mounted on the substrate in a state where they are mechanically and electrically joined together with the paste cured through a reflow process. As an apparatus which performs such screen printing, an apparatus described in JP-A-2005-205646 is known.
Meanwhile, in a screen printing process, when a substrate is lowered and separated from a mask after screen printing, the paste (flux when the paste is cream solder) adhering to the rear face of the mask may stick on the surface of the substrate, and consequently the substrate might drop out of the clampers holding the substrate. Since the screen printing apparatus disclosed in JP-A-2005-205646 provides clampers with claws which projects above the substrate, the substrate is constrained by the claws so as not to drop out from the clampers.
However, in this screen printing apparatus, since the claws are fixed to the clampers, a clearance which is equivalent to the thickness of the claws between the mask and the substrate during printing will be created. If the paste stagnated in the pattern openings enter clearance, it may overflow to other parts than a predetermined part on the surface of the substrate, and may have an adverse effect on printing quality. Therefore, in performing printing on, in particular, a fine pitch substrate which requires printing precision, the operation of removing the claws or the operation of exchanging the existing clampers with clampers with no claws is required. Moreover, there is a possibility that, if an operator contacts the claws during the maintenance of the screen printing apparatus, the claws may be damaged. Furthermore, if the claws are deformed by the contact, the clearance between the mask and the substrate may increase.